Investments in equity securities (Parenthetical) (Detail) $ in Millions	6 Months Ended		12 Months Ended
	Jun. 30, 2024 USD ($) Company		Dec. 31, 2023 USD ($) Company
Summary of Investment Holdings [Line Items]
Equity Securities, Fv-Ni	$ 284,871	[1]	$ 353,842	[2]
American Express Company [Member]
Summary of Investment Holdings [Line Items]
Equity Securities, Fv-Ni	35,100		28,400
Apple Inc. [Member]
Summary of Investment Holdings [Line Items]
Equity Securities, Fv-Ni	84,200		174,300
Bank of America Corporation [Member]
Summary of Investment Holdings [Line Items]
Equity Securities, Fv-Ni	41,100		34,800
Coca Cola Company [Member]
Summary of Investment Holdings [Line Items]
Equity Securities, Fv-Ni	25,500		23,600
Chevron Corporation [Member]
Summary of Investment Holdings [Line Items]
Equity Securities, Fv-Ni	$ 18,600		$ 18,800
Equity Securities [Member] | American Express Company, Apple Inc., Bank of America Corporation, The Coca-Cola Company and Chevron Corporation [Member] | Investment Concentration [Member]
Summary of Investment Holdings [Line Items]
Concentration percentage	72.00%		79.00%
Number of companies in concentration percentage | Company	5		5

[1] Approximately 72% of the aggregate fair value was concentrated in five companies (American Express Company – $35.1 billion; Apple Inc. – $84.2 billion; Bank of America Corporation – $41.1 billion; The Coca-Cola Company – $25.5 billion and Chevron Corporation – $18.6 billion).

[2] Approximately 79% of the aggregate fair value was concentrated in five companies (American Express Company – $28.4 billion; Apple Inc. – $174.3 billion; Bank of America Corporation – $34.8 billion; The Coca-Cola Company – $23.6 billion and Chevron Corporation – $18.8 billion).